As seen in this post viewtopic.php?f=21&t=10630, I have reviewed the hobbed bolts coming from Printrbot, Airwolf3D, and Arcol. By far the big winner is the Arcol Hyena. My recommendation is to get the chrome-plated version, not for "bling" but for longer life. The Arcol Hyena is brass, which is much softer than steel; chrome plating will make it more durable.

Hobbed Bolt Reviews

well written, thank you. my silly question is for robbed bolts - where do you use them? what I got from PBHQ was essentially a hotbed nut with a set screw. where does the "bolt" mate for torque to the extrusion motor in these cases that you show?

Welcome to the antiquities department. Hobbed bolts are used in Wade's extruders, which are geared extruders, such as all the extruders seen in this post: viewtopic.php?f=16&t=3484.

Wade's extruders - geared extruders - were pretty much necessary when plastic filament was all "3mm" in diameter. Then they came out with 1.75mm filament, which didn't require as much torque and so they started making "Direct Drive" extruders without the gear-down ratio between the motor and the driving point. So all the new Printrbots come with Direct Drive. Several of us old-timers prefer the 3mm filament and Wade's extruders. But I can understand how it would be confusing to someone who never saw, or heard of, a Wade's extruder.

That said, you can buy the hobbed "nuts" for direct drive from Arcol, called "Mini Hyena."

I believe it was Lutzbot that recently said that the demand for 3.0 and 1.75 is pretty much equal. Here in PB land, where they've gone over to the 1.75 dark side, we're shielded from reality. 1.74 direct drive extruders are easier and cheaper to manufacture. However...

I've been in contact with PB about the length/speed limitations of the AMU (misses steps after about 18mm of filament extrusion), they claim that's normal (they mentioned the U13 specifically) and a consequence of the limited melt zone (which prevents oozing, just not in Moose land), and we should just print slower. More on that to follow...